A common goal of successive generations of radio frequency communications systems is to increase the amount of information transmitted in a given communications band. As an example, US patent application publication No. 2016/0204969 discloses a Semi-Orthogonal Multiple Access (SOMA) system that allows multiple electronic devices to use superposed constellations to share wireless spectrum. In SOMA, superposed symbol constellations are formed from constituent constellations that use different modulation layers. The resulting symbols include higher modulation layer and lower modulation layer sub-symbols that have different decoding reliabilities. Higher modulation layer sub-symbols have lower reliability and can be more suitable for high signal-to-noise ratio (SNR) channels. Lower modulation layer sub-symbols have higher reliability and can be more suitable for lower SNR channels. The phrase “semi-orthogonal” in SOMA derives from the property that, from the perspective of a high SNR receiver decoding the lower reliabilty sub-symbols, the higher reliabilty sub-symbols can be considered as non-interfering and thus orthogonal to the lower reliability sub-symbols. However, from the perspective of a low SNR receiver decoding the higher reliabilty sub-symbols, the lower reliability sub-symbols can be considered as interference that is non-orthogonal to the higher reliabilty sub-symbols. SOMA may, for example, be applied in advanced radio access technologies. To date, proposed SOMA systems have relied on a QAM constellation that is the same as that specified for IEEE 802.11ac compliant radio access technologies.
Systems that can further improve on wireless spectrum usage using superposed constellations are desirable.